Circuits are known for driving print wire actuators for matrix printheads and high speed printers. These circuits may regulate current using a pedestal scheme, a chopper scheme, or an on/off type drive and are illustrated in FIGS. 1, 2 and 3, respectively.
The pedestal driver, FIG. 1, requires dual drive voltages, one high for the initial charge and a lower voltage to sustain current. A chopper type driver, FIG. 2, requires only a single drive voltage, but neither the pedestal nor chopper drivers provide pulse width control without the addition of a timing circuit, one for each actuator. Also, for the chopper driver, precautions must be taken to prevent signal noise from affecting circuit operation.
An on/off type driver, FIG. 3, provides the advantages of a single drive voltage and pulse width control but offers the drawback that it requires a resistor or diode in the flyback path in order to quickly discharge current. Current must be discharged rapidly in this scheme in order to maintain a fast actuator repetition rate. While this scheme offers significant advantages, the diode or the like in the flyback path unnecessarily wastes a significant amount of power required to fire the actuator.
U.S. Pat. No. 3,909,681 to Campari et al discloses a drive circuit for an electromagnetic coil for hammer actuation in a high speed impact printer. The driver employs two switching devices, one above the coil and one below the coil, for controlling the current. The drive circuit employs one circuit device for controlling the peak current value. Current pulse width is controlled by external logic which also initiates the start of the current pulse. The circuit is not self-timing and cannot automatically adjust current pulse width to compensate for power supply or coil impedance variations.